Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329, which is incorporated herein by reference in its entirety. The carbonylation catalyst contains a metal catalyst, such as rhodium, which is either dissolved or otherwise dispersed in a liquid reaction medium or supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. The reaction is conducted by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved.
Methanol and carbon monoxide are fed to a reactor as feedstocks. A portion of the reaction medium is continuously withdrawn and provided to a flash vessel where the product is flashed and sent as a vapor to a purification train. The purification train includes a light ends column which removes “light” or low boiling components as an overhead and provides a sidedraw stream for further purification. The purification train may further include columns to dehydrate the sidedraw stream or for removing “heavy” or high boiling components, such as propionic acid, from the sidedraw stream. It is desirable in a carbonylation process for making acetic acid to minimize the number of distillation operations to minimize energy usage in the process.
U.S. Pat. No. 5,416,237 discloses a process for the production of acetic acid by carbonylation of methanol in the presence of a rhodium carbonylation catalyst, methyl iodide and an iodide salt stabilizer by maintaining a finite concentration of water of up to about 10% by weight and a methyl acetate concentration of at least 2% by weight in the liquid reaction medium and recovering the acetic acid product by passing the liquid reaction medium through a flash zone to produce a vapor fraction which is passed to a single distillation column from which an acetic acid product is removed. The vapor fraction comprises water up to about 8% by weight, acetic acid product, propionic acid by-product and the majority of the methyl acetate and methyl iodide.
U.S. Pat. No. 7,820,855 discloses a carbonylation process for producing acetic acid including: (a) carbonylating methanol or its reactive derivatives in the presence of a Group VIII metal catalyst and methyl iodide promoter to produce a liquid reaction mixture including acetic acid, water, methyl acetate and methyl iodide; (b) feeding the liquid reaction mixture at a feed temperature to a flash vessel which is maintained at a reduced pressure; (c) heating the flash vessel while concurrently flashing the reaction mixture to produce a crude product vapor stream, wherein the reaction mixture is selected and the flow rate of the reaction mixture fed to the flash vessel as well as the amount of heat supplied to the flash vessel is controlled such that the temperature of the crude product vapor stream is maintained at a temperature less than 90° F. cooler than the feed temperature of the liquid reaction mixture to the flash vessel and the concentration of acetic acid in the crude product vapor stream is greater than 70% by weight of the crude product vapor stream. Through the flash vessel the product acetic acid and the majority of the light ends (methyl iodide, methyl acetate, and water) are separated from the reactor catalyst solution, and the crude process stream is forwarded with dissolved gases to the distillation or purification section in single stage flash. The methyl iodide concentrations decrease as the temperature of the flash vessel is increased and the flow rates are decreased.
U.S. Pat. No. 9,006,483 discloses a production process of acetic acid that seeks to inhibit the concentration of hydrogen iodide and provide a liquid-liquid separation of an overhead from a distillation column. Acetic acid is produced by distilling a mixture containing hydrogen iodide, water, acetic acid and methyl acetate in a first distillation column to form an overhead and a side cut stream or bottom stream containing acetic acid, cooling and condensing the overhead in a condenser to form separated upper and lower phases in a decanter. According to this process, a zone having a high water concentration is formed in the distillation column above the feed position of the mixture by feeding a mixture having a water concentration of not less than an effective amount to not more than 5% by weight (e.g., 0.5 to 4.5% by weight) and a methyl acetate concentration of 0.5 to 9% by weight (e.g., 0.5 to 8% by weight) as the mixture to the distillation column and distilling the mixture. In the zone having a high water concentration, hydrogen iodide is allowed to react with methyl acetate to produce methyl iodide and acetic acid.
The need remains for improved acetic acid production processes having improved separation steps, increased production capacities and lower operating costs.